The invention relates to feedback control systems. In one particular aspect, the invention relates to individual cylinder air/fuel ratio feedback control systems for internal combustion engines.
In a typical fuel injected internal combustion engine, electronically actuated fuel injectors inject fuel into the intake manifold where it is mixed with air for induction into the engine cylinders. During open loop operation, inducted air flow is measured and a corresponding amount of fuel is injected such that the intake air/fuel ratio is near a desired value.
Air/fuel ratio feedback control systems are also known for controlling the average air/fuel ratio among the cylinders. In a typical system, an exhaust gas oxygen sensor is positioned in the engine exhaust for providing a rough indication of actual air/fuel ratio. These sensors are usually switching sensors which switch between lean and rich operation. The conventional air/fuel ratio control system corrects the open loop fuel calculation in response to the exhaust gas oxygen content for maintaining the average air/fuel ratios among the cylinders around a reference value. Typically, the reference value is chosen to be within the operating window of a three-way catalytic converter (NO.sub.x, CO, and HC) for maximizing converter efficiency.
A problem with the conventional air/fuel ratio control system is that only the average air/fuel ratio among cylinders is controlled. There may be variations in the air/fuel ratio of each cylinder even though the average of all cylinders is corrected to be a desired value. Variations in fuel injector tolerances, component aging, engine thermodynamics, air/fuel mixing through the intake manifold, and variations in fluid flow into each cylinder may cause maldistribution of air/fuel ratio among each cylinder. This maldistribution results in less than optimal performance. Further, air/fuel ratio variations may cause rapid switching, referred to as buzzing, and saturation of the EGO sensor.
One approach to regulating air/fuel ratio on an individual cylinder basis is described in U.S. Pat. No. 4,483,300 issued to Hosoka et al. In this approach, small variations in a two-state switching EGO sensor are measured to, allegedly, determine fluctuations in individual cylinder air/fuel characteristics. In response to this measurement, the appropriate injector is regulated. The inventors herein contend that, at best, it is difficult to measure such small variations in the EGO output, and such measurement would have a poor signal/noise ratio. Further, the typical EGO sensor is easily saturated such that the needed signal variations may not be available.
The inventors herein have recognized that maldistribution of air/fuel ratio among the cylinders results in periodic, time variant, fluctuations in the EGO sensor output. For example, if one cylinder is offset in a rich direction, the EGO signal would periodically show a rich perturbation during a time associated with combustion in that cylinder. Accordingly, conventional feedback control techniques, which require nonperiodic inputs, are not amenable to individual cylinder air/fuel ratio control.